This invention relates to the field of systems for sensing the presence or absence of a human being in a zone of interest. More particularly, this invention relates to the field of devices of this type used to control the operation of devices requiring the application of power, such as energy control systems, alarm detectors and the like.
Devices are known which serve to sense the presence or absence of a human being from a zone of interest, such as a room, hallway, outdoor compound or the like. In general, such devices fall into one of two classes: active or passive. Passive devices typically function by sensing a threshold change in the amount of radiation sensed by the device whenever a human being enters a room or similar confined area. Active devices typically include a transmitting device for transmitting radiation of a particular frequency, such as infrared electromagnetic radiation, ultrasonic radiation or microwave radiation, along a confined beam path, and separate detector means coupled to follow-on electronics for sensing an interruption in the radiation beam which persists longer than a predetermined minimum time period and which occurs whenever an object interrupts the radiation beam path. In another type of active device, such as that disclosed in U.S. Pat. No. 4,021,679, a transmitting device propagates radiation of a predetermined frequency towards a zone of interest, such as a doorway to a room, and a sensing device detects the back reflected radiation which occurs whenever an object or a person enters this irradiated zone, the detector being coupled to follow-on electronic circuitry which activates the electrical lighting in the room in response to the detection of reflected radiation above a predetermined threshold. In this system, a separate capacitive comparator circuit is required to deactivate the lighting. Both the passive and the active types of devices noted above are more typically used as intrusion detectors, burglar alarms, or the like.
Known human presence sensing systems suffer from a number of disadvantages. Passive devices, for example, are highly sensitive to changes in the level of the preselected radiation caused by other physical events than that desired to be detected--viz, the sudden presence of a human being. Thus, a sudden change in room temperature caused by activation of a heating system, thermal conduction currents, and thermal convection currents can all trigger passive devices, which typically rely upon radiation in the infrared region. Active devices can be triggered by a wide variety of false events, among which are sudden air movements within a room, time varying air drafts, acoustic noise generated by nearby equipment, such as bells, sirens, machinery and the like, the movement of air circulating fans, the intrusion of rodents and small animals, such as cats and dogs, mechanical vibrations of the structure in which the room is located, reflection from highly polished surfaces, reduction of reflectivity of a room due to carpets and drapes, water movement in water conduits, water noise from faulty valves, RF interference, AC line transients, radar interference and cross-talk interference between adjacent sensors. Efforts to devise a human presence detector devoid of the above disadvantages have not met with success to date.